1. Field of the Invention
The present invention relates to a semiconductor light-emitting device using a compound semiconductor material and, more particularly, to a light-emitting diode using a quantum well structure in which an active region is formed as a quantum well layer.
2. Description of the Related Art
Of III-V group compound semiconductor mixed crystals except for nitrides, InGaAlP-based materials have the largest direct transition type band gap and have received a great deal of attention as light-emitting device materials in a band of 0.5 to 0.6 .mu.m. Especially, in a p-n junction type light-emitting diode (LED), having a GaAs substrate and a light-emitting portion consisting of InGaAlP lattice-matched with GaAs, high-luminance emission can be performed in the range of red to green, as compared to a conventional LED using an indirect transition type material such as GaP or GaAsP. However, even in the LED of this type, the luminous efficiency is not sufficiently high in a shorter-wavelength region (green emission).
FIG. 1 is a sectional view showing the device structure of a conventional LED having an InGaAlP light-emitting portion. Referring to FIG. 1; a lower cladding layer 2 consisting of n-InGaAlP, an active layer 3 consisting of InGaAlP, an upper cladding layer 4 consisting of p-InGaAlP, a current diffusion layer 5 consisting of p-GaAlAs and a contact layer 6 consisting of p-GaAs are sequentially stacked on an n-GaAs substrate 1. A p-side electrode 7 consisting of AuZn, and an n-side electrode 8 consisting of AuGe, are formed on the contact layer 6 and the lower surface of the n-GaAs substrate 1, respectively.
The mixed-crystal composition is set such that the energy gap of the active layer 3, consisting of InGaAlP, becomes smaller than that of the lower or upper cladding Layer 2 or 4, and a double heterostructure is formed to confine light and carriers in the active layer 3. The composition of the current diffusion layer 5, consisting of p-GaAlAs, is set such that the current diffusion layer 5 becomes substantially transparent to the emission wavelength from the active layer 3, consisting of InGaAlP.
In the structure shown in FIG. 1, when the active layer 3 was 0.2-.mu.m thick undoped In0.5(Ga.sub.1-x Al.sub.x)0.5P (x=0.4), the conductivity type was an n type, and the concentration was 1.times.10.sup.16 cm.sup.-3 to 5.times.10.sup.16 cm.sup.-3. At that time, the emission wavelength was 565 nm (green), and the luminous efficiency was 0.07% at DC 20 mA. When x was 0.3, the emission wavelength was 585 nm (yellow), and the luminous efficiency was as low as 0.4% at DC 20 mA. In this case, a characteristic merit of the use of GaP- or GaAs-based material was not necessarily observed.
On the other hand, when x was 0.2, the emission wavelength was 620 nm (orange), and the luminance efficiency was 1.5% at DC 20 mA. The luminous efficiency higher than that of GaAlAs-based material could be obtained without removing the GaAs substrate 1, serving as a light-absorbing body to the emission wavelength.
As described above, the present inventors have found by experiment that the luminous efficiency is changed in accordance with the Al composition ratio x of the active layer (Appl. Phys. Lett. 58(1991)1010). That is because the influence of a non-radiative center becomes greater when the Al composition in the crystal of the active layer is increased (J. Electron Mater 20(1991)687). Under these circumstances, attempts have been made to obtain a shorter wavelength without changing the Al composition ratio while changing the degree of atomic order as a characteristic of the InGaAlP material under growth conditions.
In addition, a method of applying a quantum well (QW) structure to the active layer is regarded as a means for effectively improving the luminous efficiency in the short-wavelength region. In the QW structure, a quantum layer, having a thickness corresponding to about the wavelength or less of the wave function of the electrons, is formed between barrier layers. The barrier layer has an energy gap larger than that of the quantum well layer and serves as a barrier against the electrons in the quantum well. The QW structure is constituted by one quantum layer (SQW: Single Quantum Well) or two or more quantum layers (MQW: Multiple Quantum Well). In the semiconductor light-emitting element using the quantum well structure as the active layer, the electron state is quantized to equivalently increase the energy gap, thereby obtaining a shorter emission wavelength.
This phenomenon is essential to the quantum well structure and is also observed in other materials such as GaAlAs-based material. In application to the InGaAlP-based light-emitting element, however, the following problem is posed. In the InGaAlP-based material, a change in the band discontinuity value on the conduction band side is small when the Al composition is changed. Hence, the resultant quantum well structure has a very shallow well on the conduction band side. Consequently, the carriers (electrons) injected into the well layer overflow to the barrier layer. The Al composition ratio of the barrier layer is relatively high, and non-radiative centers are generated due to an increase in chemically active Al composition. The overflowing electrons cause a non-radiative recombination through the non-radiative centers. A deep well can be obtained by increasing the band gap of the barrier. However, this causes an increase in Al composition, resulting in the non-radiative centers being increased in the barrier layer, and thus the carrier injection efficiency is decreased.
Therefore, in order to obtain a satisfactory effect from the quantum well structure in the light-emitting element using the InGaAlP material, the structural parameters of the quantum well structure must be optimized.
As described above, in the conventional semiconductor light-emitting device having the quantum well structure type active layer consisting of InGaAlP, the injected carriers cannot be effectively confined in the well layer, and characteristics tend to deteriorate because of dislocation, defects, and non-radiative centers.